This invention relates to a method of making polarization retaining single-mode optical fibers, and relates particularly to a method of making a preform from which such fibers can be drawn.
In many applications of single-mode optical fibers, eg. gyroscopes, sensors and the like, it is important that the propagating optical signal retain the polarization characteristics of the input light in the presence of external depolarizing perturbations. This requires the waveguide to have an azimuthal asymmetry of the refractive index profile.
A stress birefringent optical fiber having extremely good polarization retaining characteristics is disclosed in U.S. Pat. No. 4,478,489. The refractive index asymmetry is obtained by introducing into the cladding region of the fiber on opposite sides of the core longitudinally extending regions of glass having a thermal coefficient of expansion (TCE) different from that of the cladding.
Stress birefringent single-mode fibers have been fabricated by a multiple-rod-in-tube technique for several years. One of the disadvantages of this technique is the occurrence of seeds in the fiber due to the many surfaces in the preform from which the fiber is drawn. A further disadvantage is the long duration of the sealing technique that is required to prevent the low melting point stress rod material from flowing laterally between adjacent cladding rods to azimuthal regions of the fiber other than the stress rod regions. The sealing process involves traversing a flame along a rod-in-tube assembly two or three times, each traverse lasting 45 to 60 minutes. Even after subjecting the assembly to the sealing process, stress rod material sometimes leaks past adjacent cladding rods.
The aforementioned lateral leakage problem can be avoided by the method disclosed in U.S. Pat. No. 4,561,871. Longitudinally extending holes are drilled on opposite sides of the core of a single-mode fiber preform. Stress rods are inserted into the holes, and the preform is drawn into a fiber. However, it has been very difficult to drill two holes that are parallel with the core and are not skewed. Also, the rough surfaces of the holes tend to cause seeds. An object of the invention is to provide a method of making cylindrical glass articles having at least one aperture parallel to the longitudinal axis.
A preferred stress rod material comprises SiO.sub.2 doped with an amount of B.sub.2 O.sub.3 to increase the TCE to a value sufficiently above that of the cladding glass. This dopant is advantageous in that its refractive index is lower than that of silica cladding glass; B.sub.2 O.sub.3 -containing stress rods can therefore be located relatively close to the fiber core. Although polarization retaining single-mode optical fibers have been made with stress rods formed of SiO.sub.2 doped with less than 20 wt. % B.sub.2 O.sub.3, the stress rods generally contain at least 20 wt. % B.sub.2 O.sub.3 to ensure good polarization retaining properties. Stress rods formed of SiO.sub.2 doped with as little as 15 wt. % B.sub.2 O.sub.3 have a very low viscosity at fiber draw temperatures and would run from the bottom of the preform if they were not prevented from doing so. During the drawing process, the stress rod glass can also be squeezed upwardly from the top of the draw blank as the draw blank diameter is decreased in the root portion. This can alter the geometry of the core and/or the stress rods in the drawn fiber and can result in non-uniform stress on the core. A technique is needed for preventing the flow of stress rod glass from either end of the blank during the fiber drawing process. Another object of the invention is to provide a method of making a stress birefringent optical fiber containing stress rods of glass having very low viscosity at draw temperature.
Optical quality stress rods can be formed by a flame oxidation process whereby glass particles (sometimes called soot) are deposited on a temporary mandrel having a tubular handle at one end. The soot is also deposited over the end portion of the handle so that, after a sufficiently thick coating has been deposited, the porous preform and handle can be removed from the mandrel as a unit. The handle supports the porous preform during further processing. The porous preform is consolidated to a tubular glass preform which is removed from the consolidation furnace and stretched to close the aperture and form an elongated rod. When conventional techniques are employed to form rods comprising SiO.sub.2 doped with at least 15 wt. % B.sub.2 O.sub.3 the bridging layer of soot at the end of the handle tends to break; the porous preform therefore releases from the handle. The proper bridging of the soot preform to the handle is referred to as healing. Furthermore, the consolidated high expansion borosilicate glass preform can crack from the handle. In addition, a high expansion layer that forms on the aperture surface of the consolidated preform can cause the preform to break upon cooling. It is therefore an object of the invention to provide an improved method of making borosilicate rods having at least 15 wt. % B.sub.2 O.sub.3.